36 MONTANA AGRICULTURAL COLLEGE SCIENCE STUDIES. 



made to pass upon the validity of the various new species recently- 

 described from this state, as the idea of the limitation of species must 

 and ever will remain, in large part, a matter of individual judgment, 

 dependent upon the data at hand, the experience of the botanist, and 

 the relative closeness with which he desires to draw specific lines in 

 the case of the many intergrading species or the larger polymorphic 

 forms. The recent tendency toward the more critical study of 

 species and their consequent segregation should be encouraged, as 

 the basis for a better knowledge of our flora ; the more critical recon- 

 struction of species, based upon a wider knowledge of specific varia- 

 tion and the physical and biological factors of isolation tending to 

 differentiate them in their respective habitats, must be left mainly 

 to the botanist of the future, working with larger collections and aid- 

 ed by the accumulated knowledge of the present explorers. Yet, 

 there are certain causes of error in this recent tendency toward sep- 

 aration that may easily be avoided and the most important of these 

 is the failure to recognize the confluence of related species. Forms 

 intermediate in character between two other well marked species, 

 are probably mere intergrades or hybrids between these species,, 

 particularly, if these forms be few and occur only at points of contact 

 between them and such forms are neither species nor vari- 

 eties, as frequently described. Then, single aberrant specimens 

 confined to one or few localities in a well-explored flora, can hardly 

 be more than evanescent sports, soon to be swamp- 

 ed by cross breeding, unless such variation affect the organs 

 of reproduction, or give it peculiar strength to resist untoward in- 

 fluences ; a "mutation" form, according to De Vries. 



On the other hand, we may be greatly aided in our judgment 

 of a species by rembering that, aside from their difference of char- 

 acter, upon which the botanist usually basis his judgment, there will 

 also be found some factor of isolation tending to prevent the free 

 intercrossing of the two species, for sexual sterility between closely 

 related species appears to be far more rare than commonly supposed. 

 This factor of isolation may be difference in range, or altitudinal dis- 

 tribution, due to climatic or geographical barriers; difference in hab- 

 itat, due to adaption to different physical and chemical conditions of 

 growth ; or it may be adaption to different methods of fertilization, 

 to different times of blooming or to some peculiar habit of growth. 

 Cross-sterility doubtless does exist in many cases, but can not be re- 

 lied upon at all in botany as a criterion of species: it is indicated by 

 ah absence of these intergrading forms or hybrids anc), usually, by 



